Recent research on lone-actor terrorism has emphasized that many far-right attackers are guided by the doctrine of Leaderless Resistance, which holds that individual militants have a personal onus to autonomously carry out attacks. In this framework, Italy stands out because, despite its bloody history of right-wing political violence and terrorism, it has heretofore avoided, with one notable exception, any fatal lone actor attacks. This article presents a deviant case design: focusing on the exceptional case of Gianluca Casseri, the CasaPound sympathizer who went on a shooting spree in Florence in 2011, it questions theoretical assumptions concerning the non-occurrence of lone-actor terrorism by advancing a general proposition for why terrorists opt to act individually in settings where collective action is the norm. Based on first-hand information from CasaPound militants, and extensive primary data on the radicalization of Casseri, we argue that the choice between autonomous and collective violence is not only a matter of contextual constraints, personality and strategic choice. Rather, it also crucially depends on the degree of embeddedness of an individual in his or her milieu, and on the nature of the radical movement itself. The findings thus contribute to identifying the conditions that make the occurrence of lone-actor terrorism most likely, as well as the circumstances under which existing countervailing forces might fail to impede individual radicalization.